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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        // const声明的常量，也与let一样不可重复声明。
        var message = "Hello!";
        let age = 25;

        // 以下两行都会报错
        const message = "Goodbye!";
        const age = 30;
        
    </script>
</body>

</html>